The present invention relates to a non-magnetic single-component toner, to be employed in an image forming apparatus for forming an image by electrophotographic technology, for developing an electrostatic latent image on a latent image carrier of the image forming apparatus, a method of preparing the same, and an image forming apparatus using the same. More particularly, the present invention relates to a non-magnetic single-component toner composed of a large number of mother particles and a large number of external additive particles made of at least silica and titanium oxide, a method of preparing the same, and an image forming apparatus using the same.
In a conventional image forming apparatus, a photoreceptor as a latent image carrier such as a photosensitive drum or a photosensitive belt is rotatably supported to the main body of the image forming apparatus. During the image forming operation, a latent image is formed onto a photosensitive layer of the photoreceptor and, after that, is developed with toner particles to form a visible image. Then, the visible image is transferred to a recording medium. For transferring the visible image, there are a method of directly transferring the visible image to the recording medium by using a corona transfer or a transferring roller, and a method of transferring the visible image to the recording medium via an intermediate transfer member such as a transfer drum or a transfer belt, that is, transferring the visible image twice.
These methods are employed in monochrome image forming apparatuses. In addition, for a color image forming apparatus having a plurality of photoreceptors and developing devices, there is a known method transferring a plurality of unicolor images on a transfer belt or transfer drums to a recording medium such as a paper in such a manner that the respective unicolor images are sequentially superposed on each other, and then fixing these images. The apparatuses according to such a method using a belt are categorized as a tandem type, while the apparatuses according to such a method using drums are categorized as a transfer drum type. Moreover, an intermediate transferring type is also known in which respective unicolor images are sequentially primary-transferred to an intermediate transfer medium and the primary-transferred images are secondary-transferred to a recording medium such as a paper at once. Arranged on the photoreceptor used for any of the aforementioned methods is a cleaning mechanism for cleaning toner particles after developing and residual toner particles remaining on the photoreceptor after the transferring.
As toner used for such an image forming apparatus, dual-component toner composed of a developer and a magnetic carrier is generally known. Though the dual-component toner achieves relatively stable developing, the mixing ratio of the developer and the magnetic carrier is easily varied so that the maintenance for keeping the predetermined mixing ratio is required. Accordingly, magnetic single-component toner has been developed. However the magnetic single-component toner has such a problem that clear color images are not obtained due to the opacity of magnetic material thereof. Therefore, non-magnetic single-component toner has been developed as color toner. For obtaining high-quality record images with the non-magnetic single-component toner, there are problems how to improve the charging stability, the fluidity, and the endurance stability.
Conventionally, toner to be used in an image forming apparatus is surface treated by coating toner mother particles with fine particles of external additives in order to improve the charging stability, the fluidity, and the endurance stability.
Known examples of these external additives for toner are silicon dioxide (silica: SiO2), aluminium oxide (alumina: Al2O3), and titanium oxide (titania: TiO2) which have negative charging characteristics for imparting a negative polarity to mother particles. These external additives are employed alone or in combination. In this case, these external additives are normally used in combination rather than used alone in order to make full use of their characteristics.
However, such a toner using external additives of different kinds in combination has the following problems:
(1) Even though the toner is treated with eternal additives, the toner has a charge distribution because of the particle size distribution thereof. Therefore, generation of some positively charged toner particles in the toner to be used in negatively charged state is inevitable. As a result of this, in an image forming apparatus which forms images by negative charge reversal developing, the positively charged toner particles adhere to non-image portions of a latent image carrier (photoreceptor), thereby increasing the amount of cleaning toner particles. In addition, as the number of printed sheets of paper increases, the external additive particles are gradually embedded into mother particles. This means that the amount of actually effective external additive particles are reduced, leading to increase in the amount of fog toner and also decrease in the charge of toner particles. The decrease in charge allows the toner particles to scatter.
(2) When a large amount of silica is added to maintain the fluidity of the toner in order to prevent the degradation of the toner, the fixing property should be poor while the fluidity is improved.
(3) Since increase in the amount of silica makes the negative charging capacity of the toner too high. This leads to low density of printed images. To avoid this, titania and/or alumina having relatively low electric resistance are added. However, since the primary particle diameters of titania and alumina are generally small, these are embedded gradually as the number of printed sheets of paper increases. In the embedded state, these can not exhibit their effects.
(4) To obtain excellent full color toners, it is desired to prevent generation of reverse transfer toner particles as possible.
Therefore, it is proposed in Japanese Patent Unexamined Publication No. 2000-128534 to use rutile type titanium oxide, containing anatase type titanium oxide, and having a layer treated with a silane coupling agent, as an external additive. Because of existence of spindle shaped utile type titanium oxide, titanium oxide adhering to toner mother particles is prevented from being embedded in the mother particles. Because of existence anatase type titanium oxide having well affinity with the silane coupling agent, uniform coating layer of the silane coupling agent is provided onto toner mother particles. Accordingly, uniform charge distribution and stabilized charging property can be provided without reducing the triboelectric charging property. In addition, the environment dependency, the fluidity, and caking resistance can be improved. According to the toner disclosed in this publication, the aforementioned problems (1) through (4) can be somewhat resolved.
Additionally, it is proposed in Japanese Patent Unexamined Publication No. 2001-83732 to add rutile/anatase mixed crystal titanium oxide to hydrophobic silica. Accordingly, the fluidity of the toner is improved without impairing color reproducibility, and transparency, stable triboelectric charging property can be obtained irrespective of environmental conditions such as temperature, humidity, and scattering of toner particles can be prevented, thus preventing fog of toner particles on non-image portions. Also according to the toner disclosed in this publication, the aforementioned problems (1) through (3) can be somewhat resolved.
According to the toner disclosed in the aforementioned publications, external additives of titanium oxide can be prevented from being embedded in mother particles so that somewhat stable charging property can be obtained by the effect of rutile type titanium oxide and the fluidity and environmental dependency can be improved by the effect of anatase type titanium oxide. However, the rutile/anatase type titanium oxides are used only as external additives. This means that characteristics of rutile/anatase type titanium oxide, i.e. a feature that they are hardly embedded into mother particles and charge-controlling function, are not fully exhibited and that the degree of improving the stable charging property, the fluidity, and the environment dependency should be limited. That is, in order to effectively solve the aforementioned problems (1)-(4), more improvement of toner is still required.
On the other hand, Japanese Patent Unexamined Publication No. 2000-181130 discloses toner particles made of aluminum oxide-silicone dioxide combined oxide particles which are obtained by flame hydrolysis and also discloses that good fluidity of toner particles and more stable charging behavior (faster chargeability, a higher charge capacity, and permitting constant charging over time) can be provided according to the aforementioned toner particles. However, when aluminum oxide-silicone dioxide combined oxide particles are added as external additive particles to form a negatively chargeable dry type toner, the aluminum oxide components function as positively chargeable sites so as to produce reverse transfer toner particles, thereby increasing fog and thus leading to reduction in transfer efficiency.